1. Field of the Invention
The present invention relates to a semiconductor device and a process for manufacturing such a device. More specifically the present invention relates to a silicon carbide barrier layer formed between a silicon substrate and a metal layer.
2. Description of the Related Art
Most semiconductor integrated circuits (IC's) are made by using a silicon wafer or substrate in which doped regions, i.e., regions wherein an impurity is diffused or ions are implanted, are formed to make the region electrically conductive or to form a pn junction, etc., and aluminum is usually used for forming a wiring or interconnection pattern in contact with or electrically connected to such doped regions in a silicon substrate. To increase the density and speed of operation of IC's, doped regions are made much smaller, and often, shallower. For example, a typical NMOS transistor has source and drain doped regions having a depth of about 0.3 to 0.35 .mu.m and a channel length of about 1 to 2 .mu.m. To shorten the channel length to, for example, about 0.7 to 0.8 .mu.m, the depth of the source and drain regions must be less than 0.1 .mu.m, since the threshold voltages Vt.sub.TH of the source and drain are remarkably decreased due to the shortened channel length.
This causes a problem of an extraordinary diffusion of aluminum into a silicon substrate, particularly through crystal defects, thus destroying a pn junction formed between the doped region and the silicon substrate. This extraordinary diffusion of aluminum occurs easily during heat treatment process steps, even if only at a relatively low temperature, which often must be used during the manufacture of IC's, and as a result, it is not easy to obtain a reliable device having a shallow doped region in a silicon substrate.
To prevent this diffusion of aluminum into a silicon substrate, it has been proposed to insert a barrier metal layer made of tungsten, molybdenum, etc. between the silicon substrate and the aluminum electrode or wiring layer, but such a barrier metal layer is not satisfactory because the barrier metals used still react with silicon although the reactivity thereof is smaller than that of the aluminum. Moreover, a barrier metal layer is typically formed by sputtering or evaporation, but sputtering and evaporation do not provide a good coverage over a step portion of a substrate, despite the circumstance that most wiring layers or wiring layers are brought into contact with the silicon substrate through a window portion of an insulating layer formed on the silicon substrate, i.e., through a step portion. This poor coverage of the barrier metal layer sometimes causes problems.
Therefore, there is a demand for a barrier which will prevent the diffusion of aluminum into a silicon substrate and can be applied with good coverage at a step portion.
Moreover, it is not easy to form a shallow doped region having, typically, a depth of less than 100 nm, with a high reliability, and therefore, a process for forming a shallow doped region in a silicon substrate with a high reliability is also needed.
A process, per se, is known in which silicon is heated in an atmosphere containing carbon at 1,000.degree. C. to change the silicon to silicon carbide.
A process is also known in which silicon carbide is deposited on a substrate from silane, propane and hydrogen at 1,500.degree.-1,700.degree. C. (for example, H. Matunami, "SILICON CARBIDE FILMS", THIN FILMS FROM FREE ATOMS AND PARTICLES, 1985, Academic Press Inc. pp 301-324). It is also known that silicon carbide can be made electrically conductive by doping phosphorus in the silicon carbide (see F. Mieno eta, "Selective Doped Polysilicon Growth", Journal of the Electrochemical Society, Vo. 134, No. 11, November 1987, pp 2,862-2,867). These processes, however, do not concern the formation of a silicon carbide layer between a metal layer and a silicon substrate as a barrier layer and do not teach that a silicon carbide layer can be used as a barrier layer for preventing the diffusion of metal into a silicon.